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Water Splitting

Duret and Gratzel, in 2005 45 , compared the PEC response of thin films of a-Fe2O3 deposited on a TCO substrate by SP and ultrasonic spray pyrolysis (USP), a modified technique of spray pyrolysis, and found USP films exhibiting much better (Figure 13.4) PEC responses than SP films. In the USP method, a solution of 0.02 M ferric acetylacetonate in ethanol was used as the precursor, which was first pumped at a flow rate of 1 ml min 1 to the ultrasonic nozzle, connected to a 100 kHz frequency...

15.4.1 Colloidal Synthesis of 2D TiO2 Nanostructures 2D structures or those confined in their growth in only one direction have been examined due to their large surface-to-volume ratio and large aspect ratios. Taking this 2D phenomenon to the very extreme would yield a structure like single graphene sheets, which have seen broad interest over the past five years 243,244 . In our case, a brief description of colloidal techniques leading to 2D metal oxides is more appropriate for the main...

As outlined in the introduction, photocatalyzed reactions are intimately connected to the chemistry of adsorbed water, oxygen and their intermediate species. Therefore, there is a huge interest within the field of photocatalysis and related fields in exploring the intermediate steps of the reactions leading from water to 02 and vice versa. However, it is very challenging to unravel the intermediate steps by means of averaging spectroscopic techniques, since the concentrations of intermediates...

Recently, QDs have received much attention as an important component for solar-cell devices. In QD systems, electrons are confined to a small structure of the order of a nanometer, which is comparable to the Bohr length in a semiconductor. Because of this strong confinement, several novel quantum phenomena emerge, such as phonon bottlenecks and Coulomb or spin blockades. In 2001, Nozik et al. predicted a novel phenomenon in QDs, MEG 105 . When irradiated with light, several excitons can form...

A-Fe2O3, with its bandgap of 2.2 eV, is one of the most attractive materials for harvesting solar energy for hydrogen generation through the splitting of water in PEC cells. Moreover, a-Fe2O3 is a stable material, which usually does not corrode in contact with most electrolytes over a wide range of pH. Being a low-cost and readily available material, it has always been the material of choice in studies related to photosplitting of water. PEC properties of a-Fe2O3 were extensively studied in the...

In an attempt to solve this controversy, we estimated the theoretical timescale of the decoherence between the electronic states in PbSe QDs 11 . PbSe is a IV VI (14 16) semiconductor with a rock-salt structure. Its narrow bandgap is 0.27 eV at ambient temperature 115 , which means the absorption spectra of the QDs is in the visible-light region. The conduction and valence bands of PbSe QDs are formed from the p-orbital chains of Pb and Se, respectively, and the optical transition from the VB...

Schumacher et al. investigated reactively sputtered and plasma-oxidized iron-oxide films and the effect of thermal treatment, vacuum annealing and indium incorporation on their surface composition, and photoelectrochemical, impedance and spectroscopic properties 54 . Auger depth-profile data showed no detectable amount of indium incorporation in the plasma-oxidized films, while the reactively sputtered films contained 20 at indium at the front surface, decreasing to 8 in the bulk, and then...

Department of Chemical Engineering, University of California, Santa Barbara, CA, Email mcfar engineering.ucsb.edu There are an enormous number of semiconductor formulations that could be created from earth-abundant non-toxic elements however, only a relatively tiny fraction have been synthesized and studied. Unfortunately, we do not have the basic knowledge or theory to predict in advance the existence of a suitable material system or what the composition should be. Combinatorial chemistry is...

As sketched in Figure 6.1, soft X-rays and electrons interact with the electronic structure of a PEC candidate material in numerous ways. In photoelectron spectroscopy (PES), one of the fundamental techniques of surface science 4,5 , an incoming soft X-ray photon excites an electron, either from a core level or the valence band. If excited with X-rays, then PES is often called X-ray photoelectron spectroscopy (XPS), while PES with UV excitation is often also called UV photoelectron spectroscopy...

1Totalforsvarets forskningsinstitut (FOI), Umea, Sweden, Email lars.osterlund foi.se 2Dep. Engineering Sciences, The Angstrom Laboratory, Uppsala University, Uppsala, Sweden Photocatalysis is a broad research field which lies at the heart of modern sustainable technologies such as air and water cleaning, solar hydrogen production, wet solar cells, self-cleaning and antibacterial surface coatings. A detailed understanding of photocatalytic processes is not possible without explicit model studies...

Metal-oxide nanostructures have attractive properties for various applications, including alternative energy conversion and generation. Photoelectrochemical (PEC) and photovoltaic (PV) cells produced from metal oxides have potential to aid in producing low-cost and environmentally benign energy. Exploiting the unique properties of metal oxides at the nanoscale has witnessed an impressive surge in recent years. Advancement in the colloidal synthesis of these nanomaterials is quickly pushing the...

The disadvantage of one-and two-dimensional ZnO structures for use in solar cells and photocatalysis is their generally smaller surface area compared to three-dimensional structures at the same film thickness. The electrodeposition of such structures has been achieved by the use of organic structure-directing additives in the electrodeposition bath. The use of organic additives in electrodeposition has actually been known for a quite long time, but the additives have typically been used to make...

As well as photovoltaic devices, QD-sensitized metal-oxide semiconductors can be applied to solar hydrogen production water-splitting reactions. Recently, a large number of studies have been reported on this issue, in addition to nanomaterials synthesis 76 78 and general photocatalytic reactions 79,80 . In this section, recent studies regarding the application of Figure 9.20 J Vcharacteristic of a CdS QD sensitized solar cell using a 30 nm d TiO2 layer under AM 1.5 simulated solar light...

This section will highlight the investigations of metal-oxide semiconductor (SC) nanoparticles produced by colloidal techniques. In the course of producing colloidal nanoparticles, researchers have focused on controlling porosity, nanoparticle size, size distribution and assembly properties 62 64 . Three major metal-oxide nanomaterials will be discussed, TiO2, WO3 and ZnO. While this is in no way a complete collection of all available nanoparticle systems, it will demonstrate the general...

From the data presented above it is obvious that the Obr vacancies are very active sites for the dissociation of water on the r-TiO2(110) surface. Therefore, it may be logical to assume that the Obr vacancies are also active sites for the dissociation of O2 molecules. As can be seen from the STM images depicted in Figure 4.7, the Obr vacancies are indeed active centers for the dissociation of O2 molecules. The STM image depicted in Figure 4.7b was acquired after 5 L O2 exposure 1 L (Langmuir)...

Synthesizing semiconductor electrodes by electrodeposition provides many advantages, which include ease of achieving electrical contact, precise control of the microstructure and the potential for co-deposition of dopants 59 . Furthermore, large-scale superstructures and nanoscale microstructures can be created economically with electrodeposition. In 2007, Teng and Li carried out electrodeposition of iron(III) oxide films in a three-electrode system, in which a FTO glass substrate served as the...

Swift heavy ion (SHI) irradiation plays a vital role in the field of modifications of the properties of films, foils and surfaces of bulk solids. It penetrates deep into the materials and produces a long and narrow disordered zone along its trajectory 90 . SHIs lose their energy mainly by two nearly independent processes (i) elastic collisions known as nuclear energy loss (dE dx)n, which dominates at an energy of about 1 keVamu and (ii) inelastic collisions of the highly charged projectile ion...

In the following, we will demonstrate how a combination of photoelectron spectroscopy (PES) and inverse photoemission (IPES) can be used to directly derive the band-edge position in vacuum and how these values can be correlated to the electrochemical energy scale relevant for a complete PEC device 23 . The discussion will exemplarily be done for WO3, which has been discussed as a photoanode material for photoelectrochemical (PEC) hydrogen production in recent years (e.g., 24 29 ). WO3 is a...

The hydrophilicity of a TiO2 surface increases significantly on irradiation with UV light, and this change lasts for a period of several hours in the dark 3 . This phenomenon is called photoinduced hydrophilic conversion and is used in many industrial applications. Initially, the cause of this hydrophilic conversion was the dissociative adsorption of water molecules on the oxygen vacancies that are generated by UV light. Using XPS, Sakai et al. showed that a strong correlation existed between...

Single nanocrystalline undoped and Si-doped thin films of iron oxide were prepared by atmospheric pressure chemical vapor deposition (APCVD) from Fe(CO)5 and tetra-ethoxysilane (TEOS) on a fluorine-doped SnO2 substrate at 415 C by Kay, Cesar and co-workers 39,46 . The precursors were supplied by bubbling argon gas through 0.5 ml of each liquid at the rate of 11.3 ml min 1 for Fe(CO)5 and at 19.4 ml min 1 for TEOS at 25 C, metered by thermal mass-flow controllers. It was observed that lower TEOS...

In this technique, the pyrolytic decomposition of iron salts, in the form of nitrate or chlorate, sprayed on the heated substrate surface, leads to the formation of oxide thin films. The electrical, optical and structural properties of thin films have strong dependence on the stoichiometry and microstructure, as well as the level of residual stress caused by deposition technique and interaction with the substrate 31 . Fine control over the spray-deposition parameters produces films with smooth...

Open-circuit potent (increases with bandga open-circuit potent (increases with bandga Figure 1.7 Current voltage performance curve of solid state solar cell showing open circuit potential, the saturated photocurrent level and the maximum power point. The effect of this quasi-Fermi level split is extremely important to how much useable photopotential the device can generate, specifically in relation to the bandgap energy. In crystalline silicon, for example, the bandgap energy is 1.1 eV, while...

The surfaces of single crystals of rutile TiO2, such as the (001), (110) and (100), have been studied for decades as models for metal oxide. TiO2 exists in several different polymorphic forms, such as the rutile, anatase and brookite polymorphs. While brookite is not a common TiO2 phase, the anatase phase is often found in nature. In addition, because the anatase phase can be easily synthesized with high surface area many TiO2-based catalytic materials contain substantial amounts of the anatase...

Due to its high chemical reactivity and the dissolution and recrystallization reactions occurring during film growth, it is possible to obtain highly ordered arrays of ZnO crystals by electrodeposition. This is best exhibited in its heteroepitaxial growth on single crystal GaN substrates 58,83 . ZnO and GaN have the same Wurtzite structure and similar lattice constants (a 3.25 A and c 5.21 A forZnO, a 3.16 3.19 A and c 5.13 5.19 A for GaN). Because of the small lattice mismatch, ZnO is...

In the Introduction, we emphasized the importance of nonempirical first-principle calculations. However, the applicability of this method is limited to small systems. For calculations on large systems, one has to resort to an approximate or empirical methodology. The simplest description of a bulk system can be made using two-body potential functions for atoms. Despite the simplicity of this method, some systems, such as ionic compounds, can be described qualitatively, and several physical...

The research interest in nanostructured TiO2 is based on the possibilities of using the material in various applications, such as Li-ion batteries, 41 displays 42 and dye-sensitized solar cells. 43 ZnO has nearly the same bandgap and electron affinity as TiO2, making it a possible candidate as an effective dye sensitized solar cell (DSSC) semiconductor. While little work has been done on large-scale, template-free growth of TiO2 nanowires, ZnO can readily be grown in a variety of morphologies...

In a semiconducting solid, it is understood that absorption occurs through the interaction of the electromagnetic field of the photon and the intrinsic (direct gap) or induced (indirect gap) dipole moment of the solid absorber. The details of the establishment of the dipole and the associated energy gap is not fully understood in complex solids, and thus within the group of elemental building blocks there remains an extraordinarily large number of chemically possible combinations with largely...

Transition-metal nanoparticles are interesting from the perspective of both fundamental studies and applications, and in recent years specifically gold (Au) nanoparticles have attracted great attention. The main reason for this is the surprising catalytic activity of small Au nanoparticles dispersed on oxide supports for numerous reactions 97 102 , but other potential uses, such as in photocatalysis 3,103 110 and solar energy conversion 111,112 have also triggered intense research activity into...

The development of transient absorption spectroscopy driven by the improvement in highpower pulsed-laser systems has allowed time-resolved observation of physical and chemical reactions, since the technique of flash photolysis was introduced by Professor George Porter in the 1950s 39 . The principle of submicrosecond to millisecond transient absorption spectroscopy is that a sample is irradiated with a short, intense laser pulse known as a pump pulse, and then resultant changes in the optical...

5.2.1 Soft X-Ray Absorption and Emission Spectroscopy Soft X-ray absorption spectra provide information about the unoccupied states. For example, in oxygen K-edge absorption, the oxygen 1s electron is excited to empty electronic states in the carbon allotrope conduction band, and the dipole selection rule provides a tool to study locally the O2p character of these unoccupied valence bands (Figure 5.1). The atomic nature of the core hole implies elemental and site selectivity. The probability of...

The visible-light photocatalytic activity of a doped TiO2 system was first pointed out in nitrogen-doped samples 13,43 . Sulfur-doped TiO2 in either anion-doping or cation-doping form was found to be active under visible light 18,19 . However, S-doped TiO2 suffers from catalytic poisoning by the sulfate ions generated 44 . Carbon-doped TiO2 was then fabricated using several methods 45 , and visible-light photocatalytic activity was reported for these C-doped samples 15,16 . However, the...

To date, there has been no cost-effective solar photoelectrocatalyst material system identified for the production of any product, including hydrogen however, efforts in several groups are ongoing using high-throughput synthesis and screening in attempts to identify new candidate materials. The primary unmet challenge continues to be the identification of a cost-effective solar-photon absorber suitable for photoelectrocatalysis. Once the absorber material has been identified, optimization of...

Where m( ) is the phonon dispersion relation, G0 represents the Raman peak line width, whose temperature dependence has been explored in detail 63 , and p(L) is the particle size distribution. Since experimental data for the phonon dispersion for anatase is lacking, either theoretical calculations 78 or data for rutile 69 is used. As shown in Ref. 78 the phonon dispersion is positive and similar in the G X and G N direction ( 100 and 110 directions), but rather flat in the G Z direction ( 001...

Among the first studies on the electrodeposition of compounds in the 1980s was the development of methods to prepare thin films of II IV compound semiconductors 18,19 . For example, electrodeposition of cadmium sulfide from solutions of cadmium salts was carried out by electroreduction of elemental sulfur either dissolved in dimethylsulfoxide or released by decomposition of thiosulfate in water. Light absorbers in thin-film photovoltaic cells such as CdTe and Cu(In, Ga)Se2 were also prepared by...

The above example illustrates the intrinsic limitations in efficiency associated with the use of indirect optical transition semiconductors, such as WO3, as sunlight-driven photoelectrodes. Given the low absorption coefficients, a, in WO3 for photons with energies close to the bandgap, the optical penetration depth, 1 a, for light of visible wavelengths may be of the order of several micrometers 6,13 . In addition to this, the hole diffusion length, Lp, in materials with an indirect optical...

Semiconductors with bandgaps lower than 2.2 eV lack sufficient photovoltage and need an external bias potential to split water. Iron oxide (a-Fe2O3) has a bandgap of 2.2 eV, which makes it an ideal semiconductor photoelectrode, due to its capability to absorb most of the photons of the solar spectrum. However, use of iron oxide alone cannot split water, mainly due to its inappropriate energy levels with respect to the redox levels of water. The use of a narrow bandgap n-type anode in...

Figure 7.11 XANES spectra of Pt at LIII edge for Pt coated TiO2 under 351 nm light excitation at different time delays between the laser pump pulse and the x ray probe pulse. Pt(II) acetoacetonate, are also plotted. The variation of the Pt white-line peak under different delays clearly demonstrated an oxidation state change for Pt when the spectra were compared with that of the reference. The prelimary results indicated the following observations (i) Pt in Pt TiO2 catalyst was in the mixed...

Given that the photoresponse of nanocrystalline WO3 films is restricted to only the blue region of the visible spectrum, the significant photocurrents recorded under simulated solar irradiation are achieved due to high incident-photon-to-current efficiencies. As shown in Figure 12.8, the IPCE versus wavelength plots exhibit a maximum around 400 nm, and still display sizeable efficiencies up to 480 nm. The photocurrent efficiency of about 90 , attained for the photo-oxidation of water in the...

In the past, ZnO has been used in a large variety of applications 49 . For example, its wide bandgap EZnO 3.3 eV 50 and large exciton binding energy make it a promising material for UVoptoelectronic devices 49,51 . ZnO has also been suggested as a candidate material for photoelectrochemical hydrogen production PEC 52 . Since ZnO possesses similar or even better optical and electronic properties than TiO2 which has received much PEC attention in the past 53 and chapters in this book , it is...

Finding new ways to power the future by making cleaner and safer energy without creating additional CO2 in the atmosphere is one, if not simply the most important, problem facing humanity today. Indeed, meeting the current and future demand for energy, which according to various sources should double by 2050, while managing the environmental consequences of energy production and consumption, is of crucial importance nowadays, with the industrial and economic rise of highly populated countries...

In studies on nanostructured thin films, layered structures have also attracted the attention of researchers and are known to offer considerably higher phototoelectrochemical and photo-catalytic activity. A combination of small- and large-bandgap material deposited one over the other, as shown in Figure 13.11, may absorb the full solar spectrum more efficiently. If energy-band edges match at the junction, better and efficient separation of photogenerated electrons and holes is possible. Liou et...

Among the possible rutile surfaces, the surface designated with a Miller index of 110 is the most stable 29 . This surface can assume either stoichiometric or reduced forms. The stoichiometric TiO2 110 surface has oxygen atoms placed on the titanium atoms protruding from the outermost layer. The oxygen atoms bridge two titanium atoms and thus are called bridging oxygen atoms. The bridging oxygen atoms are loosely bonded to the surface and tend to be missing, depending on the external...

In those early experiments, WO3 photoanodes were illuminated either by the large spectral range output of xenon lamps or, in order to determine the spectral photoresponse, with monochromatic light of varying wavelengths whilst being polarized against a metal cathode in aqueous solutions of different pH. This resulted in photoelectrolysis of water assisted by an external bias, with hydrogen generated at the cathode and oxygen formed at the photoanode. The bandgap energies, derived from...

It had been the search for semitransparent photoanodes, which could be employed in a photoelectrolysis cell PEC , in conjunction with a photovoltaic PV cell to form a tandem device 5,15 , that led to the development of nanostructured WO3 films. Such films consist of a network of WO3 particles with sizes in the range of tens of nanometers, as shown in a typical scanning electron micrograph SEM in Figure 12.2. Due to their relatively large and evenly distributed porosity, the whole films, which...

The electrolyte composition and its pH play an important role in affecting the PEC response, onset potential and flatband potential of the semiconductor electrode in PEC cells. The rate of transfer of carriers at the junction is enhanced by proper band-edge matching of the semiconductor with the redox level of water. However, the positions of the conduction and valence bands of semiconductors are dependent on pH. An increase in pH results in a shift in the conduction- and valence-band positions...

TiO2 exists as three polymorphs, brookite, anatase and rutile. Among these, rutile is the most stable phase. Both the anatase and rutile structures are commonly used in photocatalysis applications. Figure 10.1 illustrates the crystal structures of TiO2 in the form of TiO6 octahedrons. Each octahedron can be considered as one Ti4 cation surrounded by six O2 anions. In rutile, each distorted octahedron is in contact with ten neighbor octahedrons two sharing edge oxygen pairs and eight sharing...

Prakasam et al. first synthesized a self-organized nanoporous iron III oxide thin film by potentiostatic anodization of iron foil at different anodization potentials 58 . For this purpose, 0.25 mm thick pure iron foil was used and anodized in a two-electrode system with iron foil as the anode and platinum foil as the counter-electrode. The electrolytic medium used in the study consisted of 1 HF 0.5 NH4F 0.2 0.1 M HNO3 in glycerol pH3 at 10 C. The current time behavior during the potentiostatic...

11.7.1 Dye-Sensitized Solar Cells DSSCs The photoelectrochemical properties of electrodeposited porous ZnO films have been especially investigated in view of the use of these films in DSSCs and similar solar-cell concepts. This is due to the very similar bandgap and band positions of ZnO compared to TiO2, which was used in the original concept for DSSCs. Electrodeposition of porous ZnO for ZnO films has the advantage of film preparation at relatively low temperatures, which allows film...

A-Fe2O3, known as hematite, has been a material of interest in PEC splitting of water for a long time. It is isostructural with FeTiO3, a-Al2O3, Cr2O3 and V2O3. The crystal structure of a-Fe2O3 comprises distorted hexagonal close packing of oxygen atoms with 2 3 octahedral interstices occupied by Fe. Thus, each Fe atom has six oxygen neighbors and each oxygen atom has four Fe neighbors. In 1954, Morin 1 presented the electrical properties of a-Fe2O3, and suggested two alternative models. The...

Since the discovery of the Fujishima Honda effect 1 , the surface chemistry of TiO2 has been expected to play a central role in solar hydrogen generation processes. In addition, strong photocatalytic activity 2 and photoinduced hydrophilic conversion phenomena 3 have been observed on TiO2 surfaces. A dye-sensitized TiO2 surface can be used as a building block for solar-cell devices 4 . Introduction of some impurities in TiO2 thin films induces significant changes in their magnetic 5 and...

Complete understanding of the photocatalytic reaction process on a TiO2 surface has not been established. Initially, the reaction was conjectured to involve several reactive intermediates 2,59 61 a superoxide anion O2 , hydroxyl radicals OH' , and surface-trapped electron holes h . The OH' radical was considered to be a product of the reaction between a surface hydroxyl group and a photogenerated hole However, several researchers denied the direct production of hydroxyl radicals. In reports,...